In recent years there have been developed a variety of systems utilizing hydrogen absorbing alloys that can reversibly absorb and desorb hydrogen. Among them are, for example, (a) heat utilization systems such as heat storage systems and heat pumps which use the reaction heat involved in hydrogen absorption and desorption, and (b) hydrogen absorbing systems which take advantage of such alloys to absorb a large amount of hydrogen.
Major necessary conditions for a hydrogen absorbing alloy to be used in such systems are that:
(1) Activation is easy in the initial stage of hydriding reactions; PA1 (2) The alloy has a moderate hydrogen absorption and a desorption pressure at a given operating temperature; PA1 (3) The difference between the hydrogen absorption and desorption pressures during a reversible absorption-desorption process is small; PA1 (4) The difference between the heat involved in hydrogen absorption and desorption is small; PA1 (5) The life time or durability of the alloy is long when used as a hydrogen absorbing alloy; PA1 (6) The alloy has a great hydrogen absorption capacity; PA1 (7) Raw materials are available at low cost.
Hydrogen absorbing alloys satisfying these conditions other than (4) and (5) have been proposed by the present inventors in Japanese Patent Application No. 62-252351, which discloses Zr-Mn-Co-Al quaternary alloy systems suitable for use in thermal utilization systems in a most usable temperature range, i.e. 100.degree. C.-200.degree. C.
Said hydrogen absorbing alloys made from the quaternary alloy excel other rare-earth-Ni systems, Ti-Fe systems, Zr-Mn systems, and Mg systems in property in the temperature range 100.degree. C-200.degree. C in that the quaternary systems have properties not found in the latter systems.
Yet the quaternary systems needed further improvements from the points of important requirement (4) for efficiency and (5) for durability. Regarding the condition (4), in particular, they exhibited greater hysteresis at lower temperatures. More specifically, improvement has been looked for to overcome such disadvantage encountered especially when they are to be utilized for transportation systems used over a wide temperature range.